Screw compressor

ABSTRACT

A screw compressor is for use of a screw chiller, and comprises a pair of screw rotors and a casing housing the screw rotors, a capacity control valve for varying a ratio of volume, a motor for driving the screw rotors and an inverter for varying the rotational speed of the motor. The screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve independently or combined together according to loads. The maximum efficient point in a capacity control performed solely by the inverter is set to a rotational speed side lower than the rated operation point. In a region where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.

BACKGROUND OF THE INVENTION

The present invention relates to a screw compressor, and in particular,relates to a screw compressor suitable for a screw chiller.

As a screw compressor used for a conventional refrigerating apparatus,there is the one disclosed in JP-A-59-211790. A scroll wrap compressorof JP-A-59-211790 operates to vary a ratio of volume by performing anunload control with a capacity control valve (slide valve) in thecapacity control range of 100% to 75% of a refrigerating capacity.Further, in the capacity control range of 75% to 37.5% of therefrigerating capacity, the scroll wrap compressor increases therotational speed of the screw compressor by 1.5 times by an inverter,and by using the capacity control valve in the range of 50% to 25% ofthe refrigerating capacity, can operate to vary the ratio of volume.Such operation can provide efficiency better than when operated under a100% load.

Further, as another conventional screw compressor, there is the onedisclosed in JP-A-2004-137934. The screw compressor of JP-A-2004-137934concurrently uses a rotating speed control by means of an inverter and acompression ratio control for varying the ratio of volume by changingthe end of the compression stroke by means of a variable VI valve at thetime of adjusting the capacity so that the optimum compressor efficiencyis attained depending on the operating condition of the screwcompressor.

Because the screw compressor in JP-A-59-211790 adopts an operatingmethod in which the rotational speed is increased to a constantrotational speed and a capacity control valve is used in the range of50% to 25%, there is a problem that a highly improved performance cannotbe expected because of the increase in mechanical loss due to theincreased speed and a bypass imposed by the capacity control valve.

In the screw compressors in JP-A-59-211790 and JP-A-2004-137934, nodisclosure has been made regarding the effective use of the screwcompressors as one for a screw chiller. That is, a compressor for thescrew chiller is often operated at a refrigerating capacity ratio lowerthan the 100% rated refrigerating capacity ratio, that is, in thevicinity of 80% of the rated ratio, whereas, the screw compressors inJP-A-59-211790 and JP-A-2004-137934 are controlled such that the maximumefficient point is attained at rated operation, and therefore, effectiveoperation has not been provided as the compressor for the screw chiller.

An object of the present invention is to obtain a screw compressorcapable of operation performing an effective operation for a screwchiller.

SUMMARY OF THE INVENTION

To achieve the above described object, a screw compressor for a screwchiller according to the present invention comprises a pair of screwrotors, a casing housing the screw rotors, a capacity control valve forvarying a ratio of volume, a motor for driving the screw rotors, and aninverter for varying rotational speed of the motor, wherein the screwcompressor is controlled using rotational speed control means by theinverter and mechanical capacity control means by the capacity controlvalve either independently or combined together according to a load, andwherein the maximum efficient point in a capacity control performedsolely by the inverter is set to a rotational speed side lower thanrated operation point, and in an area where the rotational speed ishigher than the maximum efficient point, the inverter solely takescontrol from a rated rotational speed to a high rotational speed side.

Specific preferable configuration examples according to the presentinvention are as follows.

(1) When an operation in the rotational speed range at or below themaximum efficient point is demanded, the rotational speed control meansby the inverter and the mechanical capacity control means by thecapacity control valve are used concurrently according to thecapacities, thereby operating such that an efficiency becomes themaximum.

(2) The capacity control valve varies a compression starting positionprovided in the casing.

(3) The maximum efficient point is set around 80% of the ratedrefrigerating capacity when the capacity control is performedindependently by the inverter.

(4) The position of the capacity control valve is controlled based onthe rotational speed of the motor according to a suction pressure, adischarge pressure, and the load of the pair of screw rotors.

(5) When an anomaly occurs on the inverter, so that the continuousoperation of the motor by the inverter is made impossible, the motor isdirectly connected to a commercial power in an emergency manner, therebythe capacity controlled operation is continued by the capacity controlvalve same as before.

According to the present invention, a screw compressor capable ofperforming an effective operation as a screw chiller can be obtained.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a screw compressor showing one embodimentof the present invention;

FIG. 2 is an operation explanatory view of a capacity control valve inFIG. 1; and

FIG. 3 is a view showing a characteristic curve of compressor efficiencyrelative to refrigerating capacity ratio in the screw compressor of FIG.1.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one embodiment of the present invention will be describedwith reference to FIGS. 1 to 3. FIG. 1 is a sectional view of a screwcompressor showing an embodiment of the present invention, FIG. 2 is anoperation explanatory view of a capacity control valve in FIG. 1, andFIG. 3 is a view showing a characteristic curve of compressor efficiencyrelative to refrigerating capacity ratio in the screw compressor of FIG.1.

A screw compressor 50 is constituted by a screw compressor for a screwchiller comprising a compressor portion 17, a motor portion 18 and acontrol device 23. Refrigerant gas to be compressed flows to thecompressor portion 17 through the motor portion 18, and after beingcompressed by the compressor portion 17, is discharged outside thecompressor. While the screw compressor 50 performing a capacity controloperates in such a manner that the rotational speed and the position ofa capacity control valve are changed so that pressure becomes constant,the control pressure is set to an arbitrary pressure.

The compressor portion 17 comprises a main casing 1, screw rotors 2, acapacity control valve 11, a rod 12, a hydraulic piston 13, a coilspring 14, a discharge casing 21, roller bearings 6 and 7, a ballbearing 8, and the like.

The main casing 1 forms a suction port 9, a discharge port 10, a gasoutlet 19, and the like. The suction port 9 forms a suction flow passagetoward the screw rotors 2, and the discharge port 10 forms a dischargepassage from the screw rotors 2, and the gas outlet 19 forms a dischargeflow passage toward the outside. The discharge casing 21 is disposedopposite to the motor of the main casing 1, and is fixed to the maincasing 1.

The screw rotors 2 are constituted by a pair of a male rotor 2A and afemale rotor (not shown) which are engaged with each other, and ishoused in a pair of cylindrical bores (not shown). Shaft portionprovided at both sides of the male rotor 2A are respectively supportedby the roller bearing 6 disposed in the main casing 1 and the rollerbearing 7 and the ball bearing 8 disposed in the discharge casing 21.

The capacity control valve 11 is for performing a capacity control bybypassing a part of refrigerant gas sucked into the engaged portion ofthe screw rotors 2 toward the suction side, and is movably housed in aconcave portion 1 b extending laterally. According to motor frequencycontrolled by the inverter 5, a position of the capacity control valve11 attaining the best efficiency is controlled. The capacity control tobypass a part of the sucked refrigerant gas to the suction side can beefficiently performed as compared with capacity control to bypass a partof discharge gas to the discharge side. The hydraulic piston 13 is apiston for driving the capacity control valve 11 left and right throughthe rod 12, and is slidably housed in a cylinder 15 which extendslaterally. The coil spring 14 is disposed at a capacity control valvechamber side of the cylinder 15, and always applies a force to press thehydraulic piston 13 toward a side opposite to the capacity controlvalve. A capacity control mechanism (mechanical capacity control means)is constituted by the capacity control valve 11, the rod 12, thehydraulic piston 13, and the coil spring 14.

The motor portion 18 comprises a motor casing 16, a driving motor 22 andthe like, and is disposed such that driving force of the motor portion18 is transmitted to the compressor portion 17. The motor casing 16 andthe main casing 1 are fixed to each other with both end surfaces thereofin closely contact with each other, and at the same time, the interiorsthereof are communicated with each other. A side surface opposite to thecompressor side of the motor casing 16 is formed with a gas inlet 20 forsucking the refrigerant gas to be compressed.

The driving motor 22 is constituted by a motor stator 3 and a motorrotor 4, and is disposed inside the motor casing 16. The motor stator 3is arranged on the inner peripheral surface of the motor casing 16. Themotor rotor 4 is fixed to the shaft portion formed at one side of themale rotor 2A, and is rotatably disposed inside the motor stator 3. Bysuch structure, the driving force of the driving motor 22 is transmittedto the male rotor 2A. Note that the female rotor is driven by the malerotor 2A.

The control device 23 comprises an inverter 5 for controlling arotational speed of the driving motor 22, and a valve control portion 26for controlling the position of the capacity control valve 11.

The inverter 5 controls the rotational frequency of the motor portion 22according to loads. The control device 23 is connected with a powersupply, a suction pressure sensor 24, and a discharge pressure sensor25. The suction pressure sensor 24 is a sensor for detecting a suctionpressure of the compressor, for example, a pressure of the gas inlet 20to input to the control device 23. The discharge pressure sensor 25 isfor detecting the discharge pressure of the compressor, for example, apressure of the gas outlet 20 to input to the control device 23.

The screw compressor 50 is constituted to be controlled using rotationalspeed control means by the inverter 5 and mechanical capacity controlmeans by the capacity control valve 11 either independently or combinedtogether according to loads. Further, the screw compressor 50 isstructured such that the maximum efficient point when an independentcapacity control is performed by the inverter 5 is set in a rotationalspeed side lower than the rated operation point and the region in whichthe rotational speed is higher than the maximum efficient point iscontrolled by the inverter alone from the rated rotational speed to thehigh rotational speed side. Further, the screw compressor 50 isstructured such that, when the operation in a rotational speed range ator below the best efficiency point is required, both of the rotationalspeed control means by the inverter 5 and the mechanical capacitycontrol means by the capacity control valve 11 are used according to thecapacities so as to perform the operation in which the efficiencybecomes the maximum.

The valve control portion 26, even when an anomaly occurs on theinverter 5 and the continuous operation by the inverter 5 is madeimpossible, directly connects the motor 22 to a commercial power in anemergency manner so as to effect a capacity control by the capacitycontrol valve 11, and performs a control such that a capacity controlledoperation by the capacity control valve 11 is continued same as before.In this manner, the operational reliability of the screw compressor 50can be improved.

In the screw compressor 50 thus structured, by supplying the power tothe driving motor 22 through the inverter 5, the driving motor 22 isrotated by the predetermined rotational speed by the inverter 5, andfurther, the compressor portion 17 is rotated. As a result, therefrigerant gas to be compressed is sucked into the motor casing 16through the gas inlet 20, and cools the driving motor 22, and thereafteris sucked into the screw rotors 2 through the suction port 9, and afterbeing compressed by the screw rotor 2, is discharged to the dischargeflow passage from the discharge port 10, and further, is discharged toan outside flow passage from the gas outlet 19.

The refrigerating cycle of the screw chiller, as shown in FIG. 1, isstructured by connecting the screw compressor 50, a condenser 27, anexpansion valve 28, and an evaporator 29 in order in annular manner.High temperature and high pressure refrigerant discharged from the screwcompressor 50 is condensed by the condenser 27 by heat exchange with theair by a fan 30 so as to become a low temperature and high pressureliquid refrigerant to be supplied to the expansion valve 28. The lowtemperature and low pressure liquid refrigerant depressurized by theexpansion valve 28 is evaporated by heat exchange with water of a coldwater piping 29 b by a refrigerant piping 29 a of the evaporator 29 soas to become a low pressure gas to be returned to the screw compressor50. The cold water cooled by the cold water piping 29 b is used forcooling control.

A temperature sensor 31 is attached to the cold water piping 29 b of theevaporator 29, and a detection signal representing a cooling watertemperature from the temperature sensor 31 is input to the controldevice 23. So, the control device 23, taking the cooling watertemperature based on the input detection signal as information from aload side, controls the inverter 5 and the capacity control valve 11 toperform capacity adjustment for the load.

In the screw compressor 50, capacity adjustment for the load is executedby inputting the signal from the suction pressure sensor 24 and thesignal from the discharge pressure sensor 25 into the control device 23,and at the same time, by inputting the signal from the temperaturesensor 31 to the control device 23, and based on these signals, byperforming the rotational speed control of the driving motor 22 by theinverter 5 and the position control of the capacity control valve 11 bythe valve control portion 26.

The capacity adjustment for the load, as described above, is performedby the control of the inverter 5 alone from the rated rotational speedto the high rotational speed side in the region where the rotationalspeed is higher than the maximum efficient point when the independentcapacity control is performed by the inverter 5, and by a combination ofthe rotational speed control by the inverter 5 and the mechanicalcapacity control by the capacity control valve 11 so that the efficiencybecomes the maximum according to the capacity in the region where therotational speed is at or below the maximum efficient point.

Here, in the region where the refrigerating capacity ratio is higherthan the refrigerating capacity ratio serving as the maximum efficientpoint (refrigerating capacity ratio of 80% of the rating in the presentembodiment), as shown in FIG. 2( a), the capacity control valve 11 ismoved to the motor side in the shaft direction so as not to bypass therefrigerant gas, and the rotational speed of the driving motor 22 iscontrolled by the inverter 5. Further, in the region where therotational speed is at or below the maximum efficient point, as shown inFIG. 2( b), the capacity control valve 11 is moved opposite to the motorside in the shaft direction so as to bypass the refrigerant gas to thesuction side, and at the same time, the rotational speed of the drivingmotor 22 is controlled by the inverter 5.

Overall adiabatic efficiency and cooling capacity of the screwcompressor 50 will be described with reference to FIG. 3. In FIG. 3, anaxis of ordinate shows compressor efficiency and an axis of abscissasshows refrigerating capacity ratio. An alternate long and short dashline in the drawing indicates an efficiency curve by the rotationcontrol by the inverter, and a broken line indicates an efficiencycurve, in case where the rotational speed is changed by the inverter andis fixed to respective rotational speeds and at the same time, thecapacity control is performed by the capacity control valve. Note that asold line in the drawing indicates an efficiency curve in case where acontrol is performed such that the rotational speed control by theinverter and the capacity control by the capacity control valve arecombined so as to reach the best efficiency.

As apparent from FIG. 3, in the case of the screw compressor 50, in thepresent embodiment, combining the control by the inverter 5 and thecontrol by the capacity control valve 11, it is possible to increase thecompressor efficiency in the refrigerating capacity ratio not more than80%, and particularly in the region where the refrigerating capacityratio is low, it is possible to sharply increase the compressorefficiency.

According to the invention, a screw compressor efficiently operated as ascrew chiller can be obtained.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A screw compressor for a screw chiller, comprising: a pair of screwrotors; a casing housing the screw rotors; a capacity control valve forvarying a ratio of volume; a motor for driving the screw rotors; and aninverter for varying a rotational speed of the motor, wherein the screwcompressor is controlled using rotational speed control means by theinverter and mechanical capacity control means by the capacity controlvalve either independently or combined together according to loads, andwherein the maximum efficient point in the capacity control performedsolely by the inverter is set to a rotational speed side lower than arated operation point, and in a region where the rotational speed ishigher than the maximum efficient point, the inverter solely takescontrol from a rated rotational speed to a high rotational speed side.2. The screw compressor according to claim 1, wherein when an operationat a rotational speed region at or below the maximum efficient point isdemanded, the rotational speed control means by the inverter and themechanical capacity control means by the capacity control valve are usedconcurrently according to capacities so that efficiency becomes themaximum.
 3. The screw compressor according to claim 1, wherein thecapacity control valve varies a compression starting position providedin the casing.
 4. The screw compressor according to claim 1, wherein themaximum efficient point is set to around 80% of a rated refrigeratingcapacity when an independent capacity control by the inverter isperformed.
 5. The screw compressor according to claim 2, wherein aposition of the capacity control valve is controlled based on therotational speed of the motor according to a suction pressure, adischarge pressure and a load of the pair of the screw rotors.
 6. Thescrew compressor according to claim 1, wherein when an anomaly occurs onthe inverter and a continuous operation of the motor by the inverter ismade impossible, the motor is directly connected to a commercial powerin an emergency manner and the capacity controlled operation by thecapacity control valve is continued same as before.